ALFRETON, DERBYSHIRE, ENGLAND — When a water treatment works in northern England faced a spike in pesticide concentration exceeding the allowable concentration limit for the incoming water, the site was forced to shut down. The company then had to divert water from a regional water treatment works in order to provide clean drinking water to its customers until the problem could be solved.

The solution arrived in the form of a mobile, lorry-mounted carbon dosing system, housed in a 6m long steel shipping container that was delivered and activated within one day, without costly and time-consuming site preparation, construction or complex components.

Supplied by Transvac Systems Ltd. (www.transvac.co.uk), the TransPAC mobile powder handling and carbon dosing system includes a bulk bag discharger and two flexible screw conveyors from Flexicon (Europe) Ltd. and Transvac's ejector system for mixing and injecting a slurry of PAC (powdered activated carbon) into the municipal water stream.

The mobile unit requires only connections to an electric power supply, the municipal water stream, and an external water supply. Environmental impact and site preparation are minimised, as well as the need for maintenance and planning permission. The system is safe to operate, and simple to control.

The water treatment works was restored to compliance as the dosed carbon successfully removed pesticide traces from the main water stream. Dosing is accurate and steady without over-dosing or wastage.

From the BFF-C-X Bulk-Out™ split-frame bulk bag discharger, PAC is automatically transferred from a half tonne bulk bag through a flexible screw conveyor to a surge hopper from which a second flexible screw conveyor meters the powder into the Transvac ejector.

The conveyors are curved to fit the tight space within the shipping container.

From the control panel, the operator sets the speeds of the conveyor drives to automatically dose the proper amount of PAC according to the site water flow. Low and high level sensors in the surge hopper signal the controller to start or stop flow through the first flexible screw conveyor when the hopper contents reach low or high level.

The carbon dosing portion of the TransPAC system includes a header tank for incoming water, a booster pump and the ejector. The velocity of the water flowing through a venturi creates a low pressure zone in the ejector that entrains the carbon powder into the treated water stream at a rate set at the control panel. The unit operates with no moving parts.

PAC poses handling problems

Powdered activated carbon adsorbs the pesticide on its surface, and the carbon and adsorbed material are subsequently removed as sludge in the flocculation process. However, the extremely fine powder, with an average particle size of only 20 microns and a bulk density of 230 kg/m3, is prone to dusting.

Both the bulk bag discharger and flexible screw conveyors prevent dusting. The bag outlet spout is connected to the feeder by a Spout-Lock™ clamp ring, which creates a secure, dust-tight connection between the clean side of the bag spout and clean side of the bag spout interface.

Each flexible screw conveyor consists of a stainless steel screw rotating inside a durable polymer tube that contains the fine powder as it is conveyed. The conveyor discharge is likewise dust-free, as powder exits through a transition adapter located forward of the drive at the discharge end, thereby preventing powder from contacting bearings or seals.

Transvac has deployed its mobile TransPAC dosing systems in a number of UK water treatment works for similar emergencies for pesticide, or taste or odour problems and as an alternative to traditional PAC batch dosing systems, which are large, complex and costly, and require long lead times by comparison.

The 5m long flexible screw conveyor (left) from the bulk bag discharger moves the carbon powder to the surge hopper, from which the second 3.5m long flexible screw conveyor moves the powder to the intake of the Transvac ejector (right).